<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box div {
            width: 400px;
            height: 400px;
            background: red;
        }

        /* 实现块元素水平布局用float */
        /* left */
        .box div:first-child {
            float: left;
        }

        .box div:last-child {
            float: right;
        }
    </style>
</head>

<body>
    <div class="box">
        <div>left</div>
        <div>right</div>
    </div>
</body>

</html>